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I recover what is left into 30lb recovery tank I use for virgin refrigerant only.When tank is almost full of virgin refrigerant I will use it when needed.
I recycle my old tanks at scrap yard or repurpose them.
Friend of mine makes bells out of tanks and sells at craft shows.

The picture in my avatar is of the Houston Ship Channel and was taken from my backyard. I like to sit outside and slap mosquitos while watching countless supertankers, barges and cargo ships of every shape and size carry all sorts of deadly toxins to and fro. It's really beautiful at times.....just don't eat the three eyed fish....

I suppose it's too much to be considered "de minimis" per the EPA? We do what shoodbefishing does. Recover into a tank clearly marked Virgin and eventually use it as virgin refrigerant. We have a set of hoses and recovery machine specifically for this task to hopefully avoid cross contamination.

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That number seems high to me. I couldn’t find the expansion rate of R-22, but propane is similar enough to R-22 that it can be used as a drop in replacement (usually not legally).

The expansion rate of liquid propane, when exposed to atmosphere, is 270 times. So 30 lbs divided by 270 is .1111 of a pound. If R-22 expands at a similar rate and the exhausted cylinder is close to atmospheric pressure, then the results should be similar, or about .1111 of a pound. I know that propane is less dense than R-22, and I know that new cylinders are not completely full of liquid, but that is quite a spread. I would have thought the answer would have been less than 2 ounces. Not saying your wrong, but just that it seems high.

There's about a 1/2 pound left, or sometimes 3/4 lb. as icemeister says. That's easy enough to see by weighing the bottle before recovering the vapor and then after punching the hole in it so the recycler will take it.

I recover what is left into 30lb recovery tank I use for virgin refrigerant only.When tank is almost full of virgin refrigerant I will use it when needed.
I recycle my old tanks at scrap yard or repurpose them.
Friend of mine makes bells out of tanks and sells at craft shows.

Same here.

I collect maybe 15 jugs, and recover the vapor. I usually get 10 - 15 lbs, because some jogs retain some small liquid during the busy times of year, and end up coming home to join the ranks until I have time in the winter to do the recovery.

There's about a 1/2 pound left, or sometimes 3/4 lb. as icemeister says. That's easy enough to see by weighing the bottle before recovering the vapor and then after punching the hole in it so the recycler will take it.

Ok, I found a conversion calculator and the expansion rate of liquid R22 is 385 times to gas at 1 atmosphere. So 30 lbs divided by 385 equals .0779 lbs.

Of course this isn’t exact because new cylinders are not completely full… the cylinder is slightly larger in order to leave room for liquid expansion, so there will be slightly more left then that.

Those referring to expansion rates from liquid to vapor at atmospheric pressure have misread the OP's question. He asked how much vapor was left in a cylinder which is at a system's low side pressure after charging.

For R22, a typical low side pressure for an A/C unit would be about 75 psig.

My number was for recovery to 15" Hg, not to full vacuum, so it should be a tad higher if you want to account for all the gas left.

To get the volume of a 30 lb cylinder I made an assumption it's only 80% full when new and used the density of R22 liquid at room temperature.

Then used the densities of R22 vapor at 75 psig and 15" Hg to get the weight of recovered vapor.

In resi work; most times a jug of 22 gets tossed when it no longer has enough pressure in it to push gas into a resi AC (or HP) system. This may well be as high as 60 or 70 PSI gauge... a lot more than atmosphere.

I think I will keep the 'empty' (that is, not enough pressure to draw from) jugs all season. Then as TB noted; in the winter recover it all into a re-usable cylinder.